Method for cleaning printing machines and printing moulds

ABSTRACT

A process is proposed for cleaning printing machines or printing plates by removing the contaminants from the surfaces to be cleaned by washing them with a microemulsion comprising water, a surfactant, and a water-immiscible organic solvent.

The invention relates to a process for cleaning printing machines andprinting plates to remove, in particular, printing inks—for example,oil-based or radiation-curable printing inks—from the cylinders androllers of printing machines, especially planographic or offset printingmachines, and from printing plates, during, for example, an interruptionin the printing process.

For said purposes it is common to employ cleaners based on organicsolvents and/or aqueous solutions. When machines are at a prolongedstandstill in printing plants, or when there is a change of ink, theparts of the printing machine that have come into contact with theprinting ink are freed from ink residues. Similarly, when there is aninterruption in the printing process, it is necessary to clean printingplates, especially planographic printing plates, carefully in order toremove ink residues and to coat them with preserving solutions based onhydrophilic polymers in order to maintain the hydrophilicity of thenonimage areas. Cleaners containing organic solvents usually havevolatile organic components (VOCs) which pollute the atmosphere and areunacceptable from an environmental and workplace health and safetystandpoint. Cleaners consisting exclusively or predominantly of apolarorganic solvents, furthermore, have the disadvantage that solventresidues which adhere to the parts to be cleaned, such as printingrollers, cannot be washed off with water after cleaning. A cleanprinting roller, however, is vital for good wetting with printing inkand for effective ink transfer. In the case of some printing plates itis also possible for the ink-carrying print stencil to undergo incipientdissolution by the cleaner and, as a result, to become damaged or evenunusable.

DE-B 27 24 557 describes a cleaner for lithographic printing plateswhich comprises water and water-miscible organic solvents. Its cleaningaction with respect to viscous oil-based printing inks is naturallylimited.

GB-A 2 089 289 describes oil-in-water and water-in-oil emulsions ascleaners. A disadvantage in this case is the relatively high interfacialtension between the water phase and the oil phase, so that, for example,lipophilic, strongly hydrophobic offset printing inks, owing to theirhigh surface energy, are taken up only slowly and minimally by thecontinuous water phase cleaner solution.

Similar comments apply to emulsions as described, for example, in WO-A90/03419 or EP-A 0 498 545.

Furthermore, emulsions of this kind are stable only kinetically but notthermodynamically, so that, especially in the case of temperaturefluctuations, they have a tendency to separate [creaming (settling),thickening, flocculation] and so are impaired in their usefulness.

A particularly difficult task is the removal of UV-curable offset orrelief inks based on polymerizable monomeric or oligomeric acrylates.They are generally removed using esters or mixtures of esters andmineral oil.

It is an object of the present invention to provide a cleaning processand a liquid cleaner (cleaning composition) which permit the rapid andeffective detachment of printing inks without polluting the localenvironment by vapors from volatile organic components or attacking theprint stencil of printing plates.

We have found that this object is achieved by a process for cleaningprinting machines or printing plates by removing the contaminants fromthe surface by washing with a liquid, wherein said liquid is apreferably bicontinuous microemulsion comprising water, a surfactantand, as the oil phase, a water-immiscible organic solvent.

For the purposes of the present specification a microemulsion is aliquid mixture, preferably a bicontinuous mixture, of water phase andoil phase with an extremely low interfacial tension between water phaseand oil phase, i.e., an interfacial tension up to three powers of tenlower than that of a conventional water-in-oil or oil-in-water emulsion.In the case of microemulsions this interfacial tension is in the rangefrom 10⁻³ to 10⁻⁷ N/m, preferably from 10⁻⁴ to 10⁻⁶ N/m, and in the caseof emulsions customarily in the range from 10⁻³ to 10⁻² N/m. Amicroemulsion in the present specification is thermodynamically stable,visually transparent and preferably of low viscosity.

Customary, conventional emulsions may comprise oil phase and water phasein very different proportions by volume. They have a continuous phaseand a disperse phase which is present as very small spherules,stabilized by coating with surfactants, in the continuous phase.Depending on the nature of the continuous phase the emulsions arereferred to as oil-in-water or water-in-oil emulsions. Ideally theseemulsions are kinetically stable, i.e., they persist for prolongedperiods although not ad infinitum. In the case of fluctuatingtemperatures in particular, they may tend toward phase separation bysettling, creaming, thickening or flocculating.

Bicontinuous microemulsions comprise two phases, a water phase and anoil phase, in the form of extended, adjacent and interpenetratingdomains at the interface between which there is an accumulation ofstabilizing surfactants in a monomolecular layer. Bicontinuousmicroemulsions form very easily, usually spontaneously on account of thevery low interfacial tension, when the individual components—water, oiland a suitable surfactant system—are mixed. Since the domains have onlyvery small extents in at least one dimension, in the order of magnitudeof nanometers, the microemulsions appear visually transparent and arestable thermodynamically, i.e., for an unlimited time, within a certaintemperature range depending on the surfactant system employed.

Bicontinuous microemulsions are described, for example, in the article“Mikroemulsionen—eine wissenschaftliche und anwendungstechnischeFundgrube?” [Microemulsions—a scientific and technological treasurechest?] by H.-F. Eicke in SÖFW-Journal 118 (1992), pages 311 to 314.

In order to achieve the required low interfacial tension at the phaseboundaries the microemulsions comprise certain amphiphiles, i.e.,surfactants, and also often comprise, in their aqueous phase, dissolvedelectrolytes and, if desired, further auxiliaries. Electrolytes areadded in particular when the amphiphiles are partly or exclusively ionicsurfactants.

The use of microemulsions to extract organic pollutants fromcontaminated soils is described in WO 94/04289. The tertiary extractionof petroleum is another known field of application for microemulsions.

In addition, EP-A-0 498 545 discloses the use of microemulsions ascleaners for surfaces such as those of coated or untreated metal panels,plastics and other surfaces, in particular for the purpose ofpretreatment for subsequent coatings.

The invention additionally provides a cleaning composition forconducting the process of the invention, said composition consisting ofa microemulsion comprising water, a surfactant and a water-immiscibleorganic solvent.

The constituents of the microemulsions should be selected such that theydo not alter the mechanical properties of components of equipment orsealing materials made of rubber or similar materials, such as theirelasticity, flexibility, dimensional stability, etc., as a result ofswelling or shrinkage (deswelling).

Water-immiscible organic solvents used are with advantage those having aboiling range above 100° C., preferably above 150° C. and, inparticular, from 200 to 400° C. In general, organic solvents havingflash points above 100° C. are employed. By organic solvents are meant,inter alia, fats and oils, such as colza oil, fatty acid esters, ethers,ketones, aldehydes, and hydrocarbons.

Generally suitable are esters, especially alkyl esters, of relativelylong-chain fatty acids. The alkyl group of the alcohol componentgenerally has 1 to 20, preferably 1 to 16 carbon atoms. The fatty acidcomponent normally has 6 to 25, preferably 8 to 18 carbon atoms and canbe linear or branched, saturated or unsaturated and contain up to threedouble bonds in the molecule. The esters generally have an iodine numberof from 0 to about 150, preferably from 0 to 40. Compounds with a higherdouble bond content frequently show a tendency toward resinification andhence toward the deposition of unwanted substances. Such compounds aretherefore added only in small amounts if at all. Examples of suitableesters are methyl, ethyl, isopropyl, n-butyl, n-hexyl, 2-ethylhexylesters and/or isooctyl esters of fatty acids or fatty acid mixtures,examples being those of octanoic, 2-ethylhexanoic, capric, lauric,myristic, palmitic, oleic, linoleic or behenic acids or of soya oil,coconut oil, palm kernel oil, palm oil, sunflower oil, sperm oil, talloil, rapeseed oil, castor oil or tallow fatty acids. Examples ofspecific typical esters are coconut fatty acid 2-ethylhexyl ester, talloil fatty acid n-hexyl ester, rapeseed oil methyl ester, methyl oleate,methyl stearate, isopropyl palmitate, ethyl laurate, 2-ethylhexyl2-ethylhexanoate, and n-octyl octanoate. In addition to these esters,ethers with a high boiling range, such as dioctyl ethers, and alsotriglycerides, such as rapeseed oil, coconut oil or soya oil, are alsosuitable.

A feature of the esters is their very low vapor pressure, so that noatmospheric pollution is caused when they are used. As is generally thecase with bicontinuous microemulsions, the proportions of aqueous andorganic phase by volume are approximately within the same order ofmagnitude; i.e., the volume ratio of water to organic phase is generallyfrom 10:90 to 90:10, preferably from 25:75 to 75:25 and, in particular,from 40:60 to 60:40.

As surfactants it is possible in principle to employ those which differin their amphiphilic nature, viz. anionic, cationic, amphoteric andnonionic surfactants or mixtures thereof.

Suitable anionic surfactants are C₁₀ to C₂₀, preferably C₁₂ to C₁₆ alkylsulfates, for example, sodium dodecyl sulfate; C₁₀ to C₂₀, preferablyC₁₂ to C₁₆ alkyl polyether sulfates, for example, sodiumdodecyloxypolyethoxysulfate; alkali metal salts ofdiisooctylsulfosuccinic acid; alkali metal salts of alkylbenzenesulfonicacids, for example, sodium dodecylbenzenesulfonate, of dialkylphosphates, and of carboxylates, for example, of fatty alkyl ethercarboxylates. Some anionic surfactants, an example being sodium dodecylsulfate, are often used together with alkanols such as butanol, pentanolor hexanol as cosurfactants and/or with alkali metal salts or alkalineearth metal salts, for example, sodium chloride or sodium sulfate orcalcium chloride, or with other electrolytes, for example NaOH, KOH,phosphates or silicates. In addition, the microemulsions employed inaccordance with the invention may also include complexing agents such asethylenediaminetetraacetic acid, nitrilotriacetic acid ormethylglycinediacetic acid, corrosion inhibitors and/or preservatives.

The alkanols can be added in amounts of up to 20% by weight, preferablyup to 10% by weight, and the electrolytes in amounts up to 10% byweight, preferably up to 5% by weight. Cationic surfactants which can beused to prepare microemulsions are, for example, alkyltrimethylammoniumhalides having alkyl chain lengths of about 8 to 18 carbon atoms and/orquaternized imidazolinium salts or pyridinium salts.

Suitable nonionic or nonionogenic surfactants are polyglycol monoalkylethers with alkyl chain lengths from C₈ to C₁₈, preferably C₁₀ to C₁₆,and from 2 to 20, preferably from 3 to 15 oxyalkylene, especiallyoxyethylene, oxypropylene and/or oxybutylene, units, or block copolymerscomprising these units. It is common to use C₁₀ to C₁₅ alkyl ethers ofpolyglycols having 3 to 10 oxyalkylene units. In the majority of casesthese are technical-grade products having a more or less broad molecularweight distribution. Surfactants having a narrow molecular weightdistribution, prepared over special catalysts, can also be employed.Also suitable are triglyceride alkoxylates, examples being reactionproducts of 1 mol of triglyceride with from 1 to 50 mol of alkyleneoxide, especially from 10 to 50 mol of ethylene oxide. It is alsopossible to use saccharide-based surfactants, examples beingalkylpolyglucosides and glucosamides.

The microemulsions employed in accordance with the invention containpreferably anionic surfactants, usually in combination with one or morenonionic surfactants. However, it is also possible to preparemicroemulsions with nonionic surfactants alone.

Achieving an optimum cleaning effect in each individual case, for eachcombination of organic solvent, surfactant or surfactants and, ifappropriate, electrolytes and complexing agents in aqueous solutionrequires defined, relatively narrow ranges of proportions of theindividual components, which can be determined by simple routineexperiments. In general, the proportion of surfactants in themicroemulsion overall is in the range from 1 to 35, preferably from 1 to25 and, in particular, from 7 to 25% by weight. Too high a proportion ofsurfactant may cause cleaning problems, or the drying of the printingrollers may present difficulties.

Proportions employed are generally from 1 to 20, preferably from 3 to 15and, in particular, from 5 to 10% by weight of anionic surfactant; from1 to 20% by weight of polyethylene glycol monoalkyl ether; from 0.1 to10, preferably from 0.5 to 5% by weight of reaction product oftriglyceride with ethylene oxide; and from 1 to 20% by weight ofpolyalkylene glycol monoalkyl ether with oxyethylene and/or oxypropyleneunits.

The microemulsions employed in accordance with the invention generallycontain from 5 to 60, preferably from 20 to 60% by weight ofwater-immiscible organic solvent and from 20 to 80, preferably from 30to 60% by weight of water. All percentages by weight here are based onthe overall weight of the finished microemulsion.

Every microemulsion is thermodynamically stable within a definedtemperature range. Preference is given to those microemulsions which arethermodynamically stable at room temperature and below. In many cases,however, it is also possible successfully to employ those microemulsionswhose stability range lies above room temperature, for example, between50 and 60° C.

High concentrations of surfactants in known cleaning liquids often leadto poor printing ink detachment in association with surfactant depositson the printing rollers; these disadvantages do not occur with themicroemulsions employed in accordance with the invention.

When the cleaning process of the invention is performed themicroemulsion is applied to those parts of the printing machine that areto be cleaned. The surface of the printing ink is wetted quickly,uniformly and completely, so that the printing ink is rapidly taken upby the cleaning liquid and dissolved or emulsified, respectively. Theresidues of microemulsion can easily be removed by washing with water.The same applies to the residues of ink which remain on a printing platewhich is to be cleaned and preserved following an interruption inprinting, especially an offset or relief printing plate. The importantfactor here is primarily the complete removal of ink residues from thenonimage or background areas of the printing plate, on which therequired hydrophilicity must be maintained, for example, in planographicor offset printing when the printing operation is resumed. Reference toa printing plate in the context of this description is to aready-to-print plate as generally obtained by exposure and developmentof a photosensitive printing plate.

The microemulsions employed in accordance with the invention are alsosuitable for cleaning other substances, such as plastics, old coatings,primers and untreated metal panels. They can be employed, for instance,as cleaners in the automotive refinish sector, and as brush cleaners.

The examples which follow illustrate embodiments of the process of theinvention, microemulsions employed in said process, and theirpreparation.

PREPARATION EXAMPLE 1

10 g of dioctylsulfosuccinate (sodium salt), 7 g of a polyglycolmonoalkyl ether mixture with about 5 oxyethylene units and a C₁₀-C₁₃alkyl ether group, 46 g of a C₈-C₁₈ fatty acid methyl ester mixture, 37g of water and 0.07 g of calcium chloride were mixed and the mixture wasshaken briefly to give a visually transparent microemulsion of lowviscosity which was thermodynamically stable at room temperature.

PREPARATION EXAMPLE 2

A microemulsion, stable at room temperature, was prepared as describedin Preparation Example 1 but from 8 g of dioctylsulfosuccinate, 16 g ofthe same polyglycol monoalkyl ether mixture, 15 g of rapeseed oil fattyacid methyl ester, 15 g of coconut fatty acid 2-ethylhexyl ester, 46 gof water and 0.07 g of calcium chloride.

PREPARATION EXAMPLE 3

14 g of dioctylsulfosuccinate, 34.5 g of soya oil and 51.5 g of waterwere mixed as in Preparation Example 1 to give a microemulsion. It wasthermodynamically stable in the temperature range from 55 to 58° C., andvisually transparent.

PREPARATION EXAMPLE 4

17.0 g of dioctylsulfosuccinate were dissolved in 41.5 g of water andthe solution was mixed with 415 g of decane. In the temperature rangefrom 51 to 56° C. the mixture forms a thermodynamically stable, visuallytransparent microemulsion of low viscosity.

Outside the stated temperature ranges the microemulsions of PreparationExamples 3 and 4 are not permanently stable and on prolonged standing atroom temperature separate into an oil phase and a water phase. Themicroemulsions of Preparation Examples 1 and 2, on the other hand, allowuse for an unlimited duration at room temperature.

USE EXAMPLE 5

In a comparative experiment the rollers of a rotary offset printingmachine were cleaned, in each case following 100,000 prints withcommercially customary oil-based offset printing ink, on the one handwith white spirit (predominantly aliphatic hydrocarbons with a boilingrange from 80 to 250° C.) and on the other with the microemulsion ofPreparation Example 1. In both cases the cleaning performance, i.e., theremoval of the printing ink, was essentially the same. When themicroemulsion was used, the rollers were cleaner and drier aftercleaning than when white spirit was used. Furthermore, the residues ofmicroemulsion were easily removable without residue, simply by rinsingthem off with water.

In the same way, the offset printing plate employed in the printingoperation was treated with both cleaning liquids. In both cases a cleanprint stencil was obtained, freed from ink residues. The printing platecleaned with the microemulsion was wetted smoothly and completely by thesubsequently applied aqueous solution of gum arabic, whereas even thenonimage area-forming support surface of the printing plate cleaned withwhite spirit accepted this solution with difficulty and only afterprolonged intensive treatment.

We claim:
 1. A process for cleaning printing machines and printingplates comprising removing contaminants from surfaces to be cleaned bywashing them with a liquid, wherein said liquid is a bicontinuousmicroemulsion comprising a) from 2 to 60% by weight of an alkylester ofa saturated or unsaturated fatty acid having 8 to 25 carbon atoms,wherein the alkyl group of said ester contains 1 to 20 carbon atoms, asa water-immiscible organic solvent, b) from 30 to 60% by weight ofwater, c) from 7 to 25% by weight of one or more surfactants, whereinthe volume ratio of water to organic phase is from 40:60 to 60:40 andthe interfacial tension between water and organic phase is in the rangefrom 10⁻⁴ to 10⁻⁶ N/m.
 2. A process as claimed in claim 1, characterizedin that said washing is conducted in a temperature range within whichthe microemulsion is thermodynamically stable.
 3. A process as claimedin claim 1, wherein said surfactant is an anionic surfactant.
 4. Aprocess as claimed in claim 3, wherein said microemulsion additionallycomprises a nonionic surfactant.
 5. A process as claimed in claim 1,wherein an electrolyte is present in solution in said water.
 6. Aprocess as claimed in claim 5, wherein said electrolyte is awater-soluble alkali metal salt or alkaline earth metal salt.
 7. Aprocess as claimed in claim 1, wherein a complexing agent or a corrosioninhibitor is present in solution in said water.